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Virtual Fossils from 425 Million-year-old Volcanic Ash

A set of exceptionally preserved but difficult-to-extract fossils reveals the diverse creatures from a Silurian sea-floor community

Derek E. Briggs, Derek J. Siveter, David J. Siveter, Mark D. Sutton

Fig.%201.%20Digging%20a%20fossil%20out%20.%20.%20.Click to Enlarge ImageWhat is a fossil? This word can mean many things, but it usually refers to the mineralized skeleton of some extinct organism—a trilobite or dinosaur, for example—which resists degradation and thus survives the eons largely intact. The fossil record of such hard parts, however, captures only a minority of invertebrates, because up to two-thirds of these species are soft-bodied—they have no shells at all. Fortunately, circumstances occasionally conspire to preserve evidence of these creatures. Here we relate such an example, one that reveals an amazing amount of detail about animals that lived during the Silurian Period.

For those who are not geologists, the time scale involved in this story may not be familiar, so first we must review some Earth history. The first major diversification of animal life took place during what paleontologists and evolutionary biologists refer to as the Cambrian Explosion. At the time, all animal life was restricted to the ocean. During much of the Cambrian Period (542–490 million years ago), most animals lacked the ability to burrow deeply into sediment. So after subsea mudflows entombed bottom-dwelling animals, their carcasses were protected from burrowing scavengers, leaving behind a fairly rich fossil record.

Deeper burrowers appeared in abundance during the succeeding period, the Ordovician, at which point buried carcasses became more vulnerable to scavenging. This is one reason why more soft-bodied animals are preserved in deposits of Cambrian age than in those from more recent times, which typically contain only fossilized hard parts.

It is for this reason that our discovery is particularly important. More than a decade ago, we found a diverse, well-preserved assemblage of largely soft-bodied fossils from the Silurian Period, which followed the Ordovician. Because they are from a typical marine setting, these remarkable fossils provide important insights into the early evolution of life in the ocean.

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